While operating in a cellular radiotelephone system, data messages are continuously transmitted between a radiotelephone and a base station. These messages include orders requesting the radiotelephone transceiver to change transmit power level, to change channel assignment, to release the call, or other similar requests. Some of these data messages are sent on the forward and reverse voice channels. In systems using subaudible data, the messages are interleaved in a continuous stream of data. A system as described in U.S. Pat. No. 4,984,290 (Levine et al.), assigned to the assignee of the present invention, is an example of such a system. Narrowband Advanced Mobile Service (NAMPS) is another example of such a system and is described in more detail in Motorola NAMPS Air Interface Specification Revision D, available from the assignee of the present invention. FIG. 1 and sections 2.7.2.2 and 3.7.2.2 of the above specification give a more detailed description of the forward and reverse voice channels. The format of the messages transmitted on both the forward and reverse channels is identical, the difference between the two channels is the data word format.
The data word format used on the narrow forward voice channel (base to radiotelephone) consists of 28 bits of content encoded into a (40,28) error correcting Bose-Chaudhuri-Hocquenghem (BCH) coded Manchester modulated data word with a distance of five sent at 100 bits/second. The BCH code allows for the correction of one bit error and is a shortened version of the standard (63,51) BCH code. The forward voice channel message is transmitted after a single transmission of a 30 bit word synchronization pattern sent at 200 bits/second using non-return to zero (NRZ) modulation. Four errors are allowed in the synchronization pattern. Bit synchronization is accomplished by receiving one of seven 24-bit continuously transmitted 200 bits/second Digital Supervisory Audio Tone (DSAT) sequences and maintaining synchronization to it. A (48,36) BCH coded Manchester modulated data word is used on the narrow reverse voice channel (radiotelephone to base). The reverse voice channel message is similarly transmitted after a single transmission of a 30 bit word synchronization pattern. Bit synchronization on the reverse voice channel is accomplished with a DSAT sequence similar to that of the forward voice channel.
Instead of multiple repeats, as used in some typical cellular systems, an automatic repeat request procedure (ARQ) is implemented for fading protection. NAMPS messages are transmitted only once with ARQ due to the low baud rate of the subaudible data. When a subaudible data word is received, an acknowledgement is sent by the receiving unit to the transmitting unit as is illustrated in FIG. 1. The message may be transmitted a total of three times if an acknowledgement is not received by the sending unit. This process is illustrated in FIG. 2.
If the radiotelephone is in a low signal level, multipath, or Rayleigh fading environment, the data word may contain uncorrectable errors when it is received. As shown in FIG. 2, if the message is received three times with uncorrectable errors, the message is ignored and no acknowledgement is sent. There is a resulting need to correct these uncorrectable errors, thereby reducing the message error rate for subaudible data communications.